Protective switching devices for monitoring the electrical current flow to an electrical load are, for example, multi-functional low-voltage switching devices within the meaning of standard EN 60947-6-2 and power switches with electronic excess current protection within the meaning of standard EN 60947-2. These are coordinated with what is referred to as a setting current (IE). This should match the rated current (IN) of the electrical load. If the current that is actually flowing exceeds the setting current a trigger signal is transmitted according to duration and flowed current. The protective switching devices either disconnect the downstream load from the supply on the occurrence of such an overload or forward the occurrence to a high-order unit. The limit values of the signal are usually detected via a trigger characteristic curve, see FIG. 1.
Three different scenarios will be considered in this case. If the current that is actually flowing exceeds the setting current (IE) by a relatively small amount, then, depending on the specific values thereof, the current is allowed to continue to flow for a while. A first region I of the characteristic curve according to FIG. 1 is used as a limit value for triggering. A protective device of this type may also be taken to mean an excess current protective device and is normatively identified as a “dependently delayed excess current trigger” to standards EN 60947-6-2 (section 5.7.1.3.2) and EN 60947-1 (section 2.4.24). This dependently delayed excess current triggering is generally used to protect users of electrical power from thermal damage.
If the current exceeds the setting current (IE) by a lot the load must be disconnected from the mains as quickly as possible. A second region II of the characteristic curve according to FIG. 1 is used as a limit value for triggering. The protective device then operates as short circuit protection and is normatively identified as an “undelayed short circuit trigger” to standards EN 60947-6-2 (section 5.7.1.3.1) and EN 60947-1 (section 2.4.26).
Owing to the need for self-protection a third region II may be defined in the transition region between short circuit protection and excess current protection in electronic protective devices, see FIG. 1. This can either be taken to mean an “independently delayed short circuit trigger” to standard EN 60947-6-2 (section 5.7.1.3.2) or an “independently delayed overload trigger” to standards EN 60947-1 (section 2.4.26) and EN 60947-1 (section 2.4.24).
Numerous advantages result if microcontrollers are used in novel electronic devices. Therefore various types of device within a connected family may be implemented using firmware variants without the hardware-side vertical range of manufacture becoming too great. With the aid of the controller it is possible to inexpensively integrate numerous special functions, the protective switching devices can be constructed so as to be adjusted more effectively and to react more accurately to the measured parameters. Furthermore, a gradual improvement in the protective switching devices is often possible without cost-intensive changes in hardware, such as what is known as a relayout, changes to tools, or the like.
However, with such protective switching devices there is the problem of an undelayed short circuit trigger and a dependently delayed excess current trigger making demands of a quite different nature.
The dependently delayed excess current trigger, also called an overload trigger, must therefore be capable of processing a corresponding I2t trigger limit according to the characteristic curve in FIG. 1. After the overload trigger has been triggered it may be activated after expiration of the restoration readiness period. The consequence of all this is that the overload trigger has to detect and evaluate current-time dependencies and periods. Brief cutoffs in the meantime must also take account of the thermal pre-stresses of the load. An advantage of this function is that the relatively large amount of time available can be seen, i.e. overload triggering is not time-critical.
As a rule the short circuit trigger does not have to detect current-time dependencies and periods. Short circuits with very low residual impedances, what are known as hard short circuits, must be detected and evaluated as quickly as possible, however. The load must be disconnected from the mains as quickly as possible in this case in order, as far as possible, to avoid consequential damages and to protect the trigger itself, what is known as self-protection. Normatively the respective rated breaking capacity of the protective switching devices is referred to which the, in part briefly very high, energy densities must withstand. The arc that is produced in the process is consumed in the arcing chambers.
This contrast between short circuit triggering and overload triggering results in quite different demands, so processing in joint groups will always be linked with compromises. A signal input filter therefore has to satisfy both demands. The resolutions required of an analog-digital converter that is used are high as an additional measuring range of the current has to be covered.
Conventional excess current protective devices contain a bimetal through which the current of the load flows. This current heats the bimetal as a function of the current intensity and duration:δQ=12R*t 
Alternatively the bimetal can also be heated by indirect heating from adjacent conductors. The different coefficients of thermal expansion of the materials in the bimetal bend the bimetal and provide a message for further processing or trigger a corresponding occurrence.
A trigger that works magnetically is used in conventional devices for the fast short circuit detection mentioned above. If the current increases then the magnetic field in a coil designed for this purpose increases. Owing to the electromagnetic forces this leads, above a certain limit value, to a movement of an armature or needle in the core of the coil. This armature conventionally triggers at a switch latch in the case of a short circuit occurrence and then causes the switch latch to unlatch or be released. In general it may therefore be said that different electromechanical components are used in conventional devices, even if both functions—overload and short circuit triggering—are present. This solution is space-intensive and expensive.
Novel devices work electronically with the aid of a microcontroller. In this case the signal characteristics of the currents of the load or the voltage characteristics are digitally sampled. This time-quantized signal is stored in discrete values. An analog-digital conversion takes place here. It is therefore possible to depict the signal characteristics more accurately and to store various characteristic curves or characteristics in the devices. Different execution algorithms result from the different demands of short circuit triggering and overload triggering. The demands on the corresponding microprocessor are correspondingly high. Mixed forms are also conceivable in which one function is conventionally implemented and the other one is electronically implemented.